1. Field of the Invention
The present invention relates to a data processing apparatus, a printing apparatus and a method for creating a mask pattern. Specifically, the present invention relates to processing in which image data is converted to dot data by using a dot arrangement pattern and then the dot data is divided by using a mask pattern into a plurality of dot data, each of which will be used for each of a plurality of times of scanning of a print head.
2. Description of the Related Art
With the diffusion of information processing equipment such as personal computers in recent years, printing apparatuses as image forming terminals have also been rapidly developing and diffusing. Of those various printing apparatuses, an ink jet printing apparatus that executes ink ejection to perform printing on a print medium such as paper, cloth, plastic sheet and OHP sheet in particular has become mainstream in regard to personal use. And this is because such an ink jet printing apparatus has excellent advantages such as being of low-noise and non-impact type printing, high-density and high-speed printing operations, easy adaptable for color printing, and of low-cost.
Advances in ink jet printing technique have been facilitated image quality improvement, faster and more economical printing, thereby contributing to the diffusion of printing apparatuses into personal users. The diffusion of personal computers and digital cameras has also contributed to the diffusion of printing apparatuses. These digital cameras include the digital camera that functions alone, as well as the digital camera that is integrated into other device, for example a mobile phone. Due to such extensive diffusion, personal users have also been requiring more improvement of image quality. Particularly, in recent years, a print system in which photographs can be readily printed at home and the printed result has an image quality comparable to silver salt photographs have been required.
In ink jet printing apparatuses, granularity has so far been seen as a problem when compared to silver salt photographs. Various measures have been proposed in order to reduce such granularity. For example, known is an inkjet printing apparatus equipped with an ink system in which light cyan and light magenta whose color material concentration are lower are added to regular cyan, magenta, yellow and black. In such an ink jet printing apparatus, the granularity can be reduced by using ink such as light cyan and light magenta in a low image density region. Meanwhile, in a high image density region, a wider color reproduction range and smooth gradation can be realized by using regular cyan and magenta inks when printing.
There is another method for reducing the granularity by designing smaller size of dots to be formed on a print medium. This can be generally realized by reducing the volume of an ink droplet to be ejected from an ejection opening of a print head. In this case, it is possible to print a high resolution image without reducing printing speed by reducing the volume of ink droplets as well as having more ejection opening at higher arrangement density.
Many approaches have been proposed for binarization processing i.e. the processing in which multiple-valued data representing the image to be printed is converted to binary data indicating whether the ink droplet should be ejected to form dots on a print medium or not. Of these approaches, for example, many types of printing apparatuses have been provided in recent years, in which binarization processing is performed in two steps in such a way that quantization processing is performed to reduce the number of gradation levels to several levels in the first step and the resulting quantized data is finally binarized in the second step. In this approach, since gradation is represented by a plurality of density levels for one pixel output from a host apparatus, the approach is preferable for application in which gradation is important, such as a photographic image quality. Furthermore, this can divide the load of data processing into two steps or processes, thus enabling suppressing the reduction of processing speed even if the amount of data to be processed is increased by the increase of printing resolution and ink color types.
Several methods have been proposed and implemented; in which data is quantized to several levels of multiple-valued data and then the multiple-valued data is converted to binary data. For example, Japanese Patent Laid-open No. 9-46522 describes a method in which a dot arrangement pattern determining the printing or nonprinting of four dots for each 2×2 area is used for one pixel that can have five levels of gradation values, to execute the binarization process. This document also describes a method in which a plurality of dot arrangement patterns for each 2×2 area are prepared for the same gradation value and these dot arrangement patterns are used sequentially or randomly. According to this method, the dot arrangement pattern for each gradation is not fixed, thus reducing pseudo outlines and the “sweeping together phenomenon” that occurs at the edge part of an image. The method also can equalize the use of a plurality of printing elements provided on a print head.
Japanese Patent Laid-open No. 2002-29097 describes reducing printing time by using a print head that has two ejection opening arrays, both arrays ejecting the same color ink droplets but each array having different characteristics each other, as well as by using these two ejection opening arrays to employ the method of switching printing/nonprinting per array (column thinning out). It also discloses the method in which a plurality of different dot arrangement patterns for the same gradation value are arranged for dealing with each of various adverse effects.
In the ink jet printing apparatuses, especially in a serial-type ink jet printing apparatus for personal users, the method referred as multi-pass printing is often employed.
FIG. 1 is a diagram illustrating the multi-pass printing and schematically showing a print head and a printing pattern by scanning of the print head. The print head is designated by a reference numeral 1001, which has 16 nozzles (ejection openings) in this Figure for simplifying the description. Sixteen nozzles are divided into four nozzle groups (first to forth groups), each nozzle group including four nozzles. A mask pattern is designated by a reference numeral 1002, indicating the pixels that enable nozzles to be used for printing (print permitting pixel; i.e. mask data area which outputs data “1” representing ejection without masking that data) as black pixels. The patterns corresponding to four nozzle groups are complementary each other, so that, by superposing these patterns, the printing in the region corresponding to 4×4 pixels can be completed.
Respective patterns designated by reference numerals 1003 to 1006 illustrate the process in which an image is being completed by repeating printing scanning. After each printing scanning, a print medium is conveyed by the width of the nozzle group in the direction of an arrow in the Figure. Thus, for the same region of the print medium (the region corresponding to the width of each nozzle group), printing of an image is completed by four times of printing scanning.
During the process of manufacturing an ink jet print head, it is unavoidable that there is a slight variation of the ejecting direction and the volume of ejected ink among a plurality of nozzles. In a serial-type printing apparatus, the amount of paper conveyed during an interval between printing scanning periods may include error in a mechanism. Such variation and error can cause adverse effects on an image such as streaks and uneven density when printing is performed by ejecting ink onto the print medium. By employing multi-pass printing described above, however, these adverse effects can be reduced. Even if there are variations in the ejection characteristics of nozzles and the amount of paper conveyed, these variations can be distributed to a plurality of times of scanning, thus making streaks and uneven density less visible. FIG. 1 shows an example of 4-pass printing in which 4 times of print scans are performed for the same image area. However, multi-pass printing is not limited to 4-pass printing. Multi-pass printing may be two-pass printing in which an image is completed by twice of print scans or may be the printing in which an image is completed by five or more times of scanning.
In addition, in the multi-pass printing, the number of dots to be printed by each printing scanning can be adjusted and the printing frequency of the nozzle that is liable to cause a problem can be reduced by devising the arrangement of a mask pattern. That is, configurations meeting various purposes can be employed apart from the purposes of preventing streaks and uneven density described above. For example, Japanese Patent Laid-open No. 2002-144552 describes masks in which the arrangement pattern of print permitting pixels of the mask is excellently dispersed. In multi-pass printing, it is known that when the printing position of a certain scan is shifted from the regular position determined relative to the printing positions of other scan, patterns (textures) by the print permitting pixels in the mask pattern applied can be visually recognized. Even in such a case, according to a mask pattern described in Japanese Patent Laid-Open No. 2002-144552, since the mask pattern that is excellently dispersed and thus visually preferable is used, the same texture as the mask pattern is not visually obtrusive or less visible, thus suppressing adverse effects on image quality.
When multi-pass printing is performed by using the dot data binarized according to the dot arrangement patterns described in Japanese Patent Laid-open Nos. 9-46522 and Nos. 2002-29097, there is a problem that uneven density occurs or a problem that the pattern of a mask pattern appears as a texture, depending on a printing image.
FIG. 2 illustrates the processing in which dot data for each of twice of scanning is created by using a mask for the image data binarized according to a dot arrangement pattern. In the Figure, pattern (a) shows four input pixels binarized according to the dot arrangement pattern as the dot arrangement pattern itself used for the binarization. One dot arrangement pattern composed of 4×2 pixels represents one gradation value by the number of dots arranged. The example in the Figure shows the four dot arrangement patterns having the same gradation value (five dots), which are composed of two species of dot arrangement patterns 401 and 402.
When mask processing is performed for the printing image expressed by these dot arrangement patterns by using masks (b) and (c) for two-pass multi-pass printing, respective dot patterns formed by respective scanning becomes patterns (d) and (e). As seen from the Figure, formed dots place a disproportionate emphasis on the pattern by one of two scanning, thus causing uneven density in a completed image. This is because the dot arrangement pattern composing a printing image and a mask pattern synchronize or interfere each other. In addition, if there is such an interference, effects of multi-pass printing for reducing variation and streaks cannot be exhibited sufficiently.
Japanese Patent Laid-open No. 5-031922 discloses one method for dealing with the similar kind of problem. This document describes that dots are thinned by using the thinning pattern of the same duty that is not synchronized with the arrangement of the specific gradation value among dot arrangements as a binary image obtained by an area coverage modulation. This can suppress the interference between the dot arrangement pattern composing an image and the mask pattern, thus preventing dots from unevenly being distributed to certain scanning.
According to the configuration for suppressing interference described in Japanese Patent Laid-open No. 5-031992, it is possible that the number of dots to be formed by each of a plurality of times of scanning is made to be equal. However, in the configuration, the relation among a plurality of dot arrangements formed by a plurality of times of scanning is not considered. Consequently, for example, the pattern of formed dots may have a certain type of geometric shape, which may make streaks and the like more visible.
As to this problem, Japanese Patent Laid-open No. 2002-144552, as described above, discloses a mask pattern in which the dispersiveness of dot arrangement is increased by mask processing. that is, the document discloses a mask pattern which makes above-mentioned streaks and the like less visible. In the mask pattern described in Japanese Patent Laid-open No. 2002-144552, however, the dot arrangement of image data to be processed by that mask is not considered. That is, only the dispersibility of print permitting pixels in the mask is considered. Therefore, especially when the image to be mask-processed has certain dot arrangement pattern such as one described in Japanese Patent Laid-open No. 9-46522 and Japanese Patent Laid-open No. 2002-29097, the effects of the dot arrangement pattern can appear in a plurality of times of scanning. In this case, equal arrangement of the number of dots between a plurality of scanning is difficult to be realized although a dispersed dot arrangement can be obtained for each scanning.
In addition, in the configuration described in Japanese Patent Laid-open No. 5-031922, if gradation is represented by a simple dot arrangement of image such as the unit of relatively small number of pixels, for example, 4×4 pixels, the mask pattern that is not synchronized with the dot arrangement pattern of an image can be easily formed. If gradation is represented by the unit of relatively large number of pixels, however, the species of dot arrangement accordingly increases, so that it is difficult to form the mask pattern that is not synchronized with the dot arrangement pattern. Furthermore, it is difficult for the method described in Japanese Patent Laid-open No. 5-031922 to deal with the case where there is a plurality of patterns for representing the same duty.